The method, step by step
Working out road base is just volume, then weight, then a buffer. Do it in that order and you cannot go far wrong.
01Measure the area. Length in metres times width in metres gives square metres. For odd shapes, break the area into rectangles and add them up.
02Pick the depth. Driveways and paths are usually 100mm to 150mm of base; a heavier vehicle area wants more. Convert millimetres to metres (150mm = 0.15m).
03Get the volume. Area times depth gives cubic metres.
04Convert to tonnes. Multiply cubic metres by about 2.1 (the compacted density of road base).
05Add 10 percent. This covers compaction, uneven ground and a bit of waste, so you are not caught short.
L (m) × W (m) × depth (m) × 2.1 × 1.1 = tonnes to order
A worked example
Say you are building a driveway base 10 metres long and 5 metres wide, at 150mm compacted depth.
| Step | Working | Result |
|---|---|---|
| Area | 10 m × 5 m | 50 m² |
| Volume | 50 m² × 0.15 m | 7.5 m³ |
| Tonnes | 7.5 m³ × 2.1 t/m³ | 15.75 t |
| Plus 10% | 15.75 t × 1.1 | ≈ 17.3 t to order |
Round up to a sensible order quantity. Ordering 18 tonnes here gives you a small, useful buffer rather than running 1 metre short on the last strip.
Do not forget the compaction allowance
This is the step that trips people up. Loose road base squashes down when it is rolled, so the depth going in is more than the finished depth. As a rough rule, 100mm of loose material compacts to about 80mm, so you lose roughly a fifth of your loose depth.
If your finished depth needs to be 150mm, you are placing closer to 185mm of loose material to get there after compaction. The 10 percent buffer in the formula partly covers this, but for deep or critical layers, design to the compacted depth and order accordingly.
Quick coverage reference
| Compacted depth | Coverage per m³ | Coverage per tonne |
|---|---|---|
| 50mm | ≈ 20 m² | ≈ 9.5 m² |
| 75mm | ≈ 13 m² | ≈ 6.3 m² |
| 100mm | ≈ 10 m² | ≈ 4.8 m² |
| 150mm | ≈ 6.5 m² | ≈ 3.1 m² |
| 200mm | ≈ 5 m² | ≈ 2.4 m² |
Based on about 2.1 t/m³ compacted. Per-tonne figures help when your supplier quotes in tonnes.